MIC 303 INDUSTRIAL AND ENVIRONMENTAL MICROBIOLOGY

1. MIC 303INDUSTRIAL AND ENVIRONMENTAL MICROBIOLOGY CHAPTER 10-SEWAGE (WASTEWATER) TREATMENT

2. Composition of Domestic Wastewater • Combination of human and animal excreta (feces and urine) and gray water resulting from washing, bathing and cooking. • Mainly composed of: • Proteins (40-60%) • Carbohydrates (25-50%) • Fats and oils (10%) • Urea derived from urine • Trace organic compound (inc pesticides, surfactants, phenols and pollutants such as non metal (As, Se), metals (Cd, Hg, Pb), benzene compounds, chlorinated compounds.

3. Overview of Wastewater Treatment • Treatment methods based on chemical and biological processes called unit processes. ↓ • Chemical unit processes include disinfection, adsorption or precipitation. • Biological unit processes involve microbial activity, responsible for organic matter degradation and removal of nutrients.

4. Wastewater Treatment Processes • Wastewater treatment comprises four steps: • Preliminary treatment: to remove debris and coarse materials that may clog equipment in the plant. • Primary treatment: treatment by physical processes such as screening and sedimentation. • Secondary treatment: Nutrient removal also generally occurs during secondary treatment of wastewater. • Biological (eg: activated sludge, trickling filter, oxidation ponds) • Chemical (e.g. disinfection) • Tertiary or advanced treatment: Biological and chemical unit processes used to further remove BOD, nutrients, pathogens and parasites and toxic substances.

5. Wastewater Treatment Processes (Con’t) • Primary treatment • Removal of solids • Disinfection • Secondary treatment • Removal of much of the BOD • Disinfection • Water can be used for irrigation • Tertiary treatment • Removal of remaining BOD, N, and P • Disinfection • Water is drinkable

6. Municipal Sewage Treatment

7. PRIMARY TREATMENT-Septic Tanks • Often use to discard domestic human waste for homes and business in areas of low population density that are not connected to municipal sewage systems. • Septic tanks: a device whose operation is similar in principle to primary treatment. • Sewage enters a holding tank and suspended solids settle out. • The sludge in the tank must be pumped out periodically and disposed off. • The effluent flows through a system of perforated piping into a leaching (soil drainage) field. • The effluent enter the soil → decomposed by soil microorganisms.

8. Septic Tanks (Con’t) Limitations: • The system work well when not overloaded and drainage system is properly sized to the load and soil type. • For heavy clay soils, require extensive drainage systems → soil’s poor permeability. • High porosity of sandy soils can results in chemical or bacterial pollution of nearby water supplies.

9. Primary Treatment: Septic Tanks

10. Oxidation Ponds • Use by small communities and industries. • Sewage (livestock waste) is dump into ponds called lagoons or stabilization ponds. • Inexpensive but require large areas of land. • Incorporate 2 stages: • Pond 1: Settle solids, pump water to pond 2 (analogous to primary treatment). The effluent will be pumped into second pond. • Pond 2: Pond is aerated by wave action. • Difficult to maintain aerobic condition for bacterial growth. However, growth of algae helps to use carbon dioxide, thus producing oxygen for bacterial consumption. • Bacterial decomposition of dissolved organic matter in water.

11. Oxidation Ponds

12. SECONDARY SEWAGE TREATMENT • Predominantly biological: • Reduce most of remain organic matter from primary treatment. • Reduce the BOD. • Sewage undergoes strong aeration → to encaurage the growth of aerobic bacteria and other microorganisms that oxidize the dissolved organic matter to carbon dioxide and water. • Two common methods: • Activated sludge systems • Trickling filters

13. Activated sludge systems • Use aeration tanks: air or pure oxygen is passed through the effluent from primary treatment. • Inoculum (Effluent from primary treatment) also termed “activated sludge”. • Activated sludge contains large numbers of sewage-metabolizing microbes, esp species of Zoogloea bacteria. • This microbial community form bacteria-containing masses in the aeration tanks called floc (sludge granules) • The soluble organic matter in the sewage is incorporated into the floc and its microorganisms. • Aeration is discontinued after 4 to 8 hours and the contents of tank are transferred to a settling tank, the floc settles out → removing much of the organic matter. • The remove solids are treated in anaerobic sludge digester (sludge didestion).

14. Activated sludge systems • The clear effluent is disinfected and discharged (Tertiary Treatment). • Activated sludge systems quite efficient → remove 75-95 % of the BOD from sewage. • Limitations: • Phenomena of “bulking” (the sludge will float rather than settle out. • The organic matter in the floc will flows out with the discharge effluent → resulting in local pollution. • Cause by filamentous bacteria (e.gSphaerolitusnatansand Nocardia sp).

15. An Activated Sludge System

16. Trickling Filters • The sewage is sprayed over a bed of rocks or molded plastic. • A biofilm of aerobic microbes grows on the rock or plastic surfaces. • Air circulates throughout the rock bed, microorganisms attached in the bioflim will oxidize organic matter trickling over the surfaces into carbon dioxide and water. • Less efficient than activated sludge system, remove 80-85 % of BOD. • Adv: less troblesome to operate and have fewer problems from overloads or toxic sewage.

17. A Trickling Filter

18. A Trickling Filter

19. Sludge Characteristics • Two classes of sludge: • Class A sludge: contains no detectable pathogens. • Class B sludge: treated to reduce numbers of pathogens below certain levels. • Can be used as fertilizers (biofertilizer) • Disadv: potential problem is contamination with heavy metals that are toxic to plants.

20. Sludge Digestion • The sludge resulted in secondary treatment often pumped to anaerobic sludge digesters. • Carried out in large tanks and oxygen is almost excluded. • Anaerobic sludge digester designed to encourage the growth of anaerobic bacteria (esp methane-producing bacteria) that degrade organic solids to soluble substances and gases, mostly methane (60-70 %) and CO2 (20-30 %). • Methane used as a fuel for heating the digester and frequently used to run power equipment in the plant. • This system resulting in a large amounts of undigested sludge, but it is stable and inert. • This sludge is pumped to shallow drying beds or water-extracting filters. • Then, it will used for landfill or as a soil conditioner (termed as biosolids).

21. Anaerobic Sludge Digester Figure 27.22

22. TERTIARY SEWAGE TREATMENT • Secondary effluent contains • Residual BOD • 50% of the original nitrogen • 70% of the original phosphorus • Tertiary treatment relies on physical and chemical methods, removes these by: • Filtration through sand and activated charcoal → remove small particulate matter and dissolved chemicals. • Chemical precipitation → Lime, alum and ferric chloride precipitate phosphate compounds. • Nitrogen is converted to ammonia and discharged into the air in stripping towers. • The purified water is the clarified and safe for drinking.